Paper products such as facial tissue, baby wipers, paper towels, toilet paper and the like are manufactured widely in the paper industry. Each of these products has unique product characteristics that require appropriate blend of product attributes to ensure that the product can be used for the intended purpose and is desired by consumers. These attributes include tensile strength, water absorbency, softness, thickness, stretch and appearance. One method of modifying and altering these properties or attributes includes providing an artistic pattern in or on the paper product. The artistic pattern typically involves a texture which is provided by either variation of density, height, or thickness variation. This texturing is generally done by a process known as embossing.
Prior art embossing processes typically involve contacting the paper product sheet with embossing equipment, which typically involve opposed rolls having a matched male and female embossing means or a metal male embossing roll and a contacting compliant (e.g., rubber) roll. The rolls operate at equal surface speeds, such that the artistic patterns of the rolls align if male and female. The web is embossed as it passes through the nip created by the two rolls.
The controls that are typically applied during embossment are the nip surface speed of the rolls, the pressure between the two rolls or nip pressure; the moisture level of the paper sheet entering the nip; the temperature of the rolls creating the nip; and the type of sheet of paper entering the nip (thickness, fiber type, smoothness, porosity, and chemical treatments). These controls affect the quality of the embossment, which is frequently judged by the clarity or sharpness of the artistic pattern on the sheet, by its uniformity across the sheet (CD or cross direction) and in the direction of motion of the sheet (MD or machine direction), and by the feel or "hand" of the embossed sheet. Adjusting these process parameters provides product variability but often results in a product without the most desirable or competitive product attributes.
It was found that rather than a single thickness or weight of tissue sheet one could dramatically change the properties of the tissue by laminating together two sheets of half the thickness or weight where each sheet had been embossed separately. The manner of laminating the two separately embossed sheets could deliver significantly different properties, softness, absorbency, feel, etc. Prior art has combined the embossing and laminating processes of separate tissue sheets into a single machine. Three different methods are currently available for commercial use for the manufacture of tissue and paper towels: 1) "Pin-on-Pin" or "Point to Point" or "peg-on-peg", 2) "Pin to Grove" or "Glued Nested"; and, 3) "Pin Embossed." The bulk or thickness and absorbency of the laminated two-ply sheet is much greater than the equivalent one-ply. This is shown, for example, by U.S. Pat. No. 3,867,225 to Nystrand.
While the Pin-on-Pin system can produce the best properties, it has associated drawbacks. Pin-on-Pin lamination of two embossed tissue sheets relies upon precise mating or alignment of the artistic patterns of the two separate male embossing elements. After the embossing nip, the two separate sheets are brought together and adhesively attached by pressing the mated protrusions of the male embossing rolls with the sheets between and adhesive between the two tissues. The mating or alignment and pressure at the location where the two male embossing rolls are closest to each other creates the bond points or bond areas of the two tissue sheets. For example, typically there is about a 0.001 inch gap set for the metal protrusions between the two metal rolls for two 20 lb. per ream sheets of tissue. As the production speed increases alignment becomes even more critical because the time of contact is shorter even though the contact forces do not diminish.
If there is even slight rotational or side-to-side misalignment with conventional Pin-on-Pin embossing/laminating, no bonding occurs and hence no acceptable product. Also, as the production speed increases, even when in a state of alignment, the sheet will stop bonding when a limiting speed is reached where vibration produces a "basket-balling" effect, i.e., the laminating rolls appear to bounce apart. This effect opens the gap between the two rolls and relieves the pressure on the bond areas before bonding can occur.
U.S. Pat. No. 3,961,119 to Thomas disclosed that some of the benefit of the Pin-on-Pin embossing/laminating could be achieved by changing from discrete pins to continuous lines for the male artistic patterns of the embossing rolls of the Pin-on-Pin process. By helical design of the line patterns on each of the separate rolls, Thomas caused the two separate bond lines to be approximately 90.degree. to each other. This produced a pinch point, square or diamond, which became a bond and precluded the need for careful alignment of the two rolls. However, this invention did not eliminate the speed limitation as it still caused undue vibration.
U.S. Pat. No. 5,173,851 to Ruppel also addressed the alignment problem by showing how an adequate level of bonding could be achieved by allowing two metal rolls to have dissimilar artistic patterns which can be discontinuous but with a prescribed regularity to produce some minimum level of contact or mating in the nip to create bonded areas of the tissue. Due to the regularity prescribed by Ruppel, the invention still had speed limitations due to deleterious vibrations.
All dynamic machinery and structures have resonant frequencies that can become problems when a regular repeated force excites the resonant condition. See, for example, "Vibration Problems in Engineering" by S. Timoshenko D. Van Nostrand Co. 1928; "Mechanical Vibrations" by William T. Thompson Prentice-Hall, Inc. 1948; "Fundamentals of Vibration Analysis" by N. O. Myklestad, McGraw-Hill 1956. A rather small regular repeating force can induce large amplitude vibration in machinery and supporting structure if the repeated force frequency is just right, i.e., equal or near to one of its critical frequencies or a harmonic of those frequencies.
To offset this adverse phenomena most dynamic machinery is installed with vibration isolation pads or dampers to prevent or mitigate the transmission of deleterious vibrations to other parts of the machinery or supporting structure. Motor mounts and automobile shock absorbers are traditional examples of this. Without shock absorbers the regular repeating force of the paved roadway expansion joints can cause an automobile to bounce wildly and go out of control. This condition does not occur until the automobile has reached or come close to the speed at which these regularly spaced small force pulses are at or near the critical frequency of the automobile suspension system.
Rotating machinery parts are balanced to preclude vibration forces from any small eccentric weight distribution. This is seen in counter weights used on automobile tires and automobile drive shafts. Another method of reducing vibrations includes creating a stiffer, more massive structure to increase the resonant frequency and preclude vibration-induced resonance from being transmitted to the structure or item to be isolated. This is typified by large massive foundation blocks for delicate instruments and for rotating machinery like compressors or turbines. Some machinery can be operated above the critical rotating frequency if one quickly passes through the critical range before the mass can reach a deleterious amplitude of vibration. Some unbalanced machinery vibrates at slow rotational speeds but when it changes from rotation about its geometric center to its dynamic center of inertia the vibration ceases.
The contact point pattern or bonding pattern created by "pin-on-pin" embossing and laminating can be assessed as to its potential for inducing a resonant vibration into the laminating nip rolls. During the roll rotation, the pinch point or pinch region of the nip-where the two sheets are compressed together to produce the lamination bond-produces opposing forces in the rolls. These forces are generally perpendicular to the axis of the roll and tend to open the gap of the nip. If the embossing rolls are an artistic pattern of many dots in regular spacing in both directions, one can readily determine the relative magnitude of the total separating force on the laminating nip of the rolls. This is done by looking at a narrow band of the laminating nip (CD band) at an instant in time, and by measuring the bonding pressure in the laminating nip. By totaling the bond areas multiplied by the bonding pressure of the simultaneous bonding regions of the laminating nip across this narrow band in the CD one can obtain a relative measure of the size of the force at a specific instant in time. The reaction forces normally varies between the supporting bearings of the two embossing rolls and the center point of the rolls. This can be corrected by crowning of the rolls specifically to create a uniform pressure at each bonding point or region of the nip across its width. The centroid of these forces can also be determined to see if it also creates a torsional moment on the rolls. After a small angle of rotation of the two metal laminating rolls, one can calculate the force at the next narrow band of the laminating nip. One can repeat this for 360.degree. of rotation and plot the time history of the force that would be acting to separate the embossing rolls at their nip over one complete revolution. These bonding or pinch points have been plotted for several embossing roll patterns as shown in FIGS. 1-5. These plots are the sum of the bonding point areas from scanning across the pattern in a narrow width corresponding to the nip width, approximately 1/20 inch at 512 successive adjacent positions to the width of about 12.5 inches.
FIG. 1 shows a commercial embossing/laminating system with oval pins at regular 1/8 inch spacing on 20 inch diameter embossing rolls. At a machine speed of 1000 ft/minute, a force pulse of 31,500 units is produced about every 0.63 milliseconds (1600 hertz), or one pulse for each row.
FIGS. 2-4 show forces versus time plots for the traditional patterns known as Ruppel, Floral Oval, and Sparkle, respectively. The regularity of these bonding patterns are revealed in the force versus time plot with a cycle time or period of less than one revolution. For example the pattern disclosed by Ruppel as shown in FIG. 2 repeats about every 7.0 rows or 4.5 milliseconds between force pulses, or a force frequency of about 224 hertz. The relative magnitude of the force, which is considered to be related to the area of contact between the rolls, is the difference between the peak and the valley of the plot or 26,000 force units.
FIG. 5 shows the forces versus time plot for an irregular pattern according to principles of the present invention. As can be seen, the relative magnitude of forces are lower than those forces produced by regular patterns. In addition, due to the irregularity of the bonding pattern, there is less repeating forces thereby reducing the damage caused by repetitive vibrations.
Therefore there exists a need for a pin-on-pin embossing/laminating process to maintain adequate bonding that is capable of achieving high speeds without resonate vibration being induced by the mated lamination (i.e., bonding points) of the two embossing patterns.